That's too big a jump. Besides, v3 is still on its shakedown cruise and will have a lot of bugs for a while. It would be better to move to v2.3 for now, and think about v3 in a year or two, after it's settled down.

1. Fulll backup existing database by export by SQL.
2. Create a new database(e.g. Upgrade) and import by SQL.
3. Make a copy of your existing Opencart folder and copy to new location(e.g. Upgrade).
4. Open config.php and admin/config.php from your new location and change the location path, DB name and db_driver(mysqli).
5. Create a subdomain(e.g. upgrade.myopencart.com) and point to your new location.
6. Test and make sure new created site are working.
(MAKE SURE YOU CHANGED THE NEW DB's NAME, you can test it by adding new record in upgrade Opencart)

7. Overwrite your upgraded folder by Opencart 3.0 files(all files and folders from upload folder).
8. Run the installation again and Opencart should detected an upgarde.

You may see this error, I just ignored and continue.
Error Code(0): Error: Unknown table 'nwtco859_upgrade.affiliate_activity'
Error No: 1051
DROP TABLE `affiliate_activity` in /home/nwtco859/upgrade/system/library/db/mysqli.php on line 40

After upgrade, clear any cookies in your browser to avoid getting token errors.

Load the admin page & press Ctrl+F5 twice to force the browser to update the css changes.
Opencart may ask you to move your storage directory.
So you move the storage directory by following the instructions, automatic Move doesn't work for me, I have to manually move the folder.
After you moved, you have to change to change the config files.

I followed these steps:
1) Created a copy of the current 1.5 files into a sub-folder.
2) Created a new database and imported the old database tables.
3) Modified the Config files.
4) Created a sub-domain
5) Copied the 3.0 upload files over top of the files in the sub-folder.
6) Ran the upgraded.domain.com/install
7) It said it was successful and then redirected me to the root of the sub-domain with an error message that a theme had not been selected. I setup a theme.

Result - I get an updated OpenCart but my system does not have the categories or products from my old site. So far this upgrade process has been worthless. Is there no automated way to update an older site? Or are there manual SQL files I can use to update the older SQL DB to the new format?

Well, if an automated way of upgrading an OpenCart would be possible, then only,
if a fully virgin old Version would have to be upgraded. Anything else is unpro-dreaming,
like expexting a WIN-10 upgrade from WIN-7 also upgrading MS-Word, Excel, and other
additional Software at the same time.

Nevermind. I didn't realize that the old tables had a prefix so they were not getting upgraded. I had to change the prefix to match the standard oc_. That's a bummer. Changing the prefix would be better from a security perspective.

1. Fulll backup existing database by export by SQL.
2. Create a new database(e.g. Upgrade) and import by SQL.
3. Make a copy of your existing Opencart folder and copy to new location(e.g. Upgrade).
4. Open config.php and admin/config.php from your new location and change the location path, DB name and db_driver(mysqli).
5. Create a subdomain(e.g. upgrade.myopencart.com) and point to your new location.
6. Test and make sure new created site are working.
(MAKE SURE YOU CHANGED THE NEW DB's NAME, you can test it by adding new record in upgrade Opencart)

7. Overwrite your upgraded folder by Opencart 3.0 files(all files and folders from upload folder).
8. Run the installation again and Opencart should detected an upgarde.

You may see this error, I just ignored and continue.
Error Code(0): Error: Unknown table 'nwtco859_upgrade.affiliate_activity'
Error No: 1051
DROP TABLE `affiliate_activity` in /home/nwtco859/upgrade/system/library/db/mysqli.php on line 40

After upgrade, clear any cookies in your browser to avoid getting token errors.

Load the admin page & press Ctrl+F5 twice to force the browser to update the css changes.
Opencart may ask you to move your storage directory.
So you move the storage directory by following the instructions, automatic Move doesn't work for me, I have to manually move the folder.
After you moved, you have to change to change the config files.

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data OK Warning: file(C:/xampp/htdocs/admin/config.php): failed to open stream: No such file or directory in C:\xampp\htdocs\install\model\upgrade\1009.php on line 94Warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\install\model\upgrade\1009.php on line 96Warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\install\model\upgrade\1009.php on line 107Warning: fopen(C:/xampp/htdocs/admin/config.php): failed to open stream: No such file or directory in C:\xampp\htdocs\install\model\upgrade\1009.php on line 116Warning: fwrite() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\install\model\upgrade\1009.php on line 118Warning: fclose() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\install\model\upgrade\1009.php on line 120{"success":"Patch 1009 has been applied (10 of 10)","next":"index.php?route=upgrade\/upgrade\/next&step=11"}

Since its July, 2018 I am wondering if anyone has successfully achieved this upgrade directly? Or if I would be better off in 2 steps:
Step 1: v1.5.6 to v2.3.02
Step 2: v2.3.02 to v3.0.2
I do not see any official documentation on how to perform this upgrade. Any suggestions for this rookie, before I attempt this upgrade? This would be my first OpenCart upgrade attempt.

Well, better don't even try, I guarantee, that you will end up in frustration.
It has been mentioned already hundreds of times ....
But probably, it's of no use, to tell all those forever Newbies ...
Ernie